1. Field of the Invention
The present invention relates to a high-efficiency encoder for use in transmission and/or recording of audio or video data.
2. Description of the Prior Art
It is known that video data which contains a large volume of information is high-efficiency encoded to a low-bit code form for ease of transmission or storage. The high-efficiency encoding involves compression of video or audio data through removing its redundant components. In common, a block encoding technique is best employed in which a sampled data input is divided into blocks and each block which contains a plurality of consecutive sample values is orthogonally transformed and compressed.
The orthogonal transformation is generally carried out by a discrete cosine transformation technique (abbreviated to as DCT technique hereinafter). It is also known that a method of orthogonal data, produced through the DCT technique, being quantized and encoded into variable length codes for transmission is most appropriate for use in the high-efficiency encoding. The variable length code encoding employs a two-dimensional variable length code encoding technique in which quantized data of each block are translated into low-bit code forms through on detection of a quantized value representing zero, both the number of zeros and the amplitude of non-zeros following the zeros are encoded simultaneously. Also, the lowest bit of each code word encoded by the variable length code encoding is commonly assigned to a sign bit denoting whether the value of the cord word is positive or negative.
The disadvantage of the variable length code encoding is such that if any error bit exists, the synchronism of code word blocks may be interrupted, thus causing decoding errors. Hence, a technique is used in which a particular string of bits is allocated to the end of each block so that it appears at no other locations throughout the succession of code words in the block. In operation, whenever the synchronism of code word blocks is disturbed, the end of each block is systematically identified by detecting the particular bit string and thus, the code synchronism can be restored.
Table 1 illustrates an example of code words in which the two-dimensional vector form of an input signal represents at right half the number of consecutive zeros and at left half the absolute value of the number of non-zeros following the zeros. More specifically, the vector (2, 1) indicates that two consecutive quantized values are 0 and followed by other quantized values whose absolute value is 1. Also, the EOB (end of block) is arranged to be replaced with the remaining quantized values, which all are zero, of the DCT block. In other words, EOB is a sign word denoting the end of a block. The lowest bit s of each code word is a sign bit denoting whether the amplitude of quantized values after a zero bit is positive or negative. In common, if it is positive, the sign bit is expressed as "0" and if negative, as 1".
TABLE 1 ______________________________________ Input signal Code word ______________________________________ (0, 1) 1 0 s (0, 2) 1 1 0 s (1, 1) 0 0 1 s (2, 1) 0 1 1 0 s (3, 1) 0 1 1 1 0 s (4, 1) 0 1 1 1 1 0 s E O B 1 1 1 1 0 s ______________________________________
As shown in Table 1, the string of six bits, 11110s, is arranged to appear at no location other than the end of a given series of code words in a block. If the code synchronism is disturbed by a transmission error, 11110s is first detected. The succeeding bit after 11110s is always the most significant bit (MSB) of a code word in the next block and thus, the code synchronism can be restored. EOB is the sign word of each DCT block and when expressed by 11110s as shown in Table 1, can permit the code synchronism to be restored with more certainty.
However, the EOB in Table 1 which contains the sign bit s at end, is substantially designated to associate with neither a positive nor a negative sign. More specifically, the EOB contains one unnecessary bit which tends to reduce the effectiveness in high-efficiency encoding. Also, if the least significant bit (LSB) s of the EOB is omitted, the restoring of code synchronism by the detection of EOB will hardly be conducted.
It is thus an object of the present invention to overcome the foregoing drawback of a known variable length encoder.